The mechanism of transmission blocking immunity to Plasmodium gallinaceum in chickens and Aedes aegypti mosquitoes has been examined. Three classes of antibody dependent mechanism one involving complement and two without have been defined. Using radiolabelled preparations of the target cells of this immunity, the gameters, zygotes and ookinetee which occurs in the mosquito midgut, it has been possible to define by immunochemical methods several target antigens. Antibody independent killing of midgut stages of the parasite by the vertebrate host's alternative pathway of complement has been shown. Parasites survive the complement systems of their natural host by a combination of temporary specific resistance coupled with complement inactivation mediated by the mosquito proteases released into the blood meal. Considerable diversity in such properties as drug sensitivity and gametocyte production has been demonstrated among clones of P. falciparu. The properties are stable in each clone. Different types of clones representing different combinations of such properties may occur in a single isolate of parasites.